Answer: Papillary endothelial hyperplasia

Histology: Within the lamina propria of the bladder is an area of recent hemorrhage. Focally within the area of recent hemorrhage there are anastomosing channels filled with blood. Papillae which appear to be primarily composed of fibrin line the spaces. The cells lining the fibrin papillae are slightly enlarged with vesicular nuclei and some visible nucleoli yet they are not pleomorphic.

Discussion: Papillary endothelial hyperplasia was first described by Masson as “vegetant intravascular hemangioendothelioma.” It is now recognized that this is not a neoplasm but rather a peculiar recanalization of a thrombus. Most cases occur within vessels and may be engrafted on preexisting vascular lesions such as hemangiomas or vascular malformations. The key differential diagnosis is angiosarcoma. It is relatively rare for papillary endothelial hyperplasia to occur extravascularly as a result of organization of a hematoma. However, this appears to be the case in the current specimen. In contrast to angiosarcoma, the cells lining the fibrin papillae lack significant pleomorphism or mitotic activity, and there is no evidence of necrosis. Where angiosarcoma appears to dissect native fibrous tissue, in the current case of papillary endothelial hyperplasia the endothelial cells are lining fibrin, which then merge into adjacent recent hemorrhage. Glomerulations within interstitial cystitis represent small punctate hemorrhages following distention of the bladder with air. These glomerulations are minute hemorrhages which would not be as large as seen in the current case and would not undergo papillary endothelial hyperplasia.